The present invention relates to an apparatus for the layer-by-layer production of three-dimensional objects, to processes for layer-by-layer production, and also to corresponding mouldings.
The rapid provision of prototypes is a task frequently encountered in very recent times. Processes which permit the rapid provision of a prototypes are described as rapid prototyping, rapid manufacturing, or as an additive fabrication process. Particularly suitable processes the rapid provision of a prototype are based on selective melting and solidification of pulverulent materials which are produced in a layer-by-layer manner through selective melting and solidification of the pulverulent material according to a pattern of the prototype object under construction. Supportive structures for overhangs and undercuts are not required in such a process, because the powder bed surrounding the molten regions provides adequate support. Correspondingly, as no supports are employed, a subsequent task of removing supports is not required. These processes are also suitable for short-run production. The temperature of the construction chamber is selected in such a way as to avoid distortion of the structures produced layer-by-layer during the construction process.
One process which has particularly good suitability for rapid prototyping/rapid manufacturing is selective laser sintering (SLS). In this process, plastics powders in a chamber are selectively briefly exposed to a laser beam, and the powder particles impacted by the laser beam therefore melt. The molten particles coalesce and rapidly solidify again to give a solid mass. This process can produce three-dimensional structures simply and rapidly by constantly applying new layers and repeatedly exposing them to laser light to melt and subsequently coalesce in the form of the three dimensional object.
The laser sintering (rapid prototyping) process for producing mouldings from pulverulent polymers is described in detail in the U.S. Pat. No. 6,136,948 and WO 96/06881 (both DTM Corporation). A wide variety of polymers and copolymers is claimed for this application, and includes polyacetate, polypropylene, polyethylene, ionomers and polyamide, for example.
Other processes with good suitability are the Selective Inhibition of Bonding (SIB) process as described in WO 01/38061 and in EP 1015214. Both processes operate with large-surface-area infrared heating to melt the powder. The selectivity of the melting process is achieved in the first case by applying an inhibitor, and in the second process by a mask. DE 10311438 describes a further process, wherein the energy needed for the fusion process is introduced via a microwave generator, and the selectivity is achieved by applying a susceptor. WO 2005/105412 describes a method where the required energy for the fusion process is introduced via electromagnetic radiation, and the selectivity is achieved by applying an absorber.
A common problem incurred with the above described conventional processes is that constituents of the polymeric powder are released or vaporized when the polymeric powders are heated. The substances or vapours which are released disrupt the process, because they condense on important components of the production apparatus, such as lenses, pyrometer or mask, and impair their function.
The constituents released often involve monomers, oligomers or additives which have a markedly lower melting point than the polymer. The content of monomers and oligomers in the polymer can be reduced, but this incurs considerable additional costs. Furthermore, in most cases the polymer still comprises residues of substances which can be released as vapours upon heating.